This invention relates to an error correction system for a multichannel parallel information handling system and, more particularly, to plural channel error correction using signal quality pointers and correcting signals from fewer than such plurality of channels without such quality pointers.
In data handling systems, information is encoded for error detection and correction purposes by adding redundant bits to the data message in such a way that the total message can be decoded with an economical apparatus to faithfully supply the original information even when plural first errors occur in such message. Parallel data arrangements, that is, arrangements where the information is contained in parallel bytes arranged within a block of data, are used in computers and are well known, especially in multichannel recording apparatus. U.S. Pat. No. 3,629,824, filed Feb. 12, 1970, discloses encoding and decoding apparatus in which the redundant or check bits are associated with the data in a cross-byte or cross-track direction. This patent sets forth a code capable of correcting one or more errors within one byte of data having a given number of bits. The data is divided into a plurality of fixed-sized signal sets each consisting of k bytes of data (each byte having b bits), plus two check bytes, each of b bits. The decoder recovers the data without error when not more than a single byte of the received message is in error no matter how many bits may be in error in the single byte. Co-pending U.S. application, Ser. No. 99,490, filed Dec. 18, 1970, and now U.S. Pat. No. 3,697,948, utilizes the above-identified code, but extends the capabilities thereof by combining therewith pointer signals which extend the error correcting capability of the arrangement to two bytes in error regardless of the number of bits in error in each byte. These systems require two channels for the two additional check bytes needed for error correction, respectively. As the density of the information along the tracks or channels has increased, a faster, more reliable, simpler, but powerful, error correcting scheme is required which utilizes only one additional track for check bits.
In one-half inch magnetic tape systems, it is highly desirable that tape be readable in both directions of transport. Usually, the tape is recorded only when transported in a first direction, arbitrarily defined as forward. A tape recorder should read in the forward and backward directions. When this fact is coupled with error detection and correction requirements, it is apparent that error codes should be operable for both directions of data transfer. Since the bit sequences are unalike in such transfers, many error detection and correction schemes require the data be accumulated before performing the error functions. For controlling costs and enhancing data throughput, it is desirable to perform error encoding and syndrome generation during readback on a serial basis--that is, perform calculations concurrently with data transfer rather than wait for all data transfers to be completed.